Low gloss powder coatings have been used for forming lusterless coating films. Up to date, low gloss powder coating compositions based on polyester, in which the gloss value of the coating is reduced by incorporating an extender pigment, such as silica, talc or the like, have been known. These powder coating compositions reduce the gloss value by the formation of minute surface irregularities on the coating film due to the incorporation of the extender pigment.
In such powder coating compositions, however, the gloss value will not be lowered down to 50 or less, even when the content of the extender pigment in the coating composition is increased over 30% by weight. In addition, the resulting coating film becomes hard and brittle together with a quite inferior appearance, when such a high content of the extender pigment is employed.
A powder coating composition for glossless coating based on polyester resins has been known, in which the above-mentioned defects were obviated by compounding two polyester resins exhibiting different curing reaction rates (Japanese Patent Application Kokai Nos. 154771/1988, 1770/1989, 98671/1989 and 109468/1991). This powder coating composition realizes the reduction of gloss value by forming a fine wrinkle pattern over the coating surface due to a difference in the rate of curing reaction of the polyester with the hardener existing, between the two polyester resins incorporated and permits the coating to reach a gloss value of below 10.
However, these conventional powder coating compositions suffer from a large fluctuation in the gloss value caused even by a small difference in, for example, the mixing proportion of the components, the condition of baking of the resulting coating and so on. In the industrial production of such powder coating compositions, occurrence of fluctuation in the mixing proportion cannot be avoided, resulting in considerable unequalities between the production lots, so that it is difficult to maintain a definite gloss value requested by users. Thereto adds further a problem that coating films of the same gloss value are difficultly obtainable even using the same coating composition, if different baking conditions are to be employed.